Selenium rectifier including tellurium and method of making it



J. N. SHIVE Aug. 26, 1952 SELENIUM RECTIFIER, INCLUDING TELLURIUM ANDMETHOD OF MAKING IT Filed Aug. 17, 1949 2 Sl-IEETS--SHEET 1 IN 5 N TORBy J. M SH VE A TTORNE V Aug. 26, 1952 J. N. SHIVE 2,608,611

SELENIUM RECTIFIER, INCLUDING TELLURIUM AND METHOD OF MAKING IT FiledAug. 17, 1949 2 SHEETS-SHEET 2 A T TOR/V5 V Patented Aug. 26, 1952SELENIUM RECTIFIER INCLUDING TEL- V V .LURIUM AND METHOD OF MAKING ITJohn N. Shive, Plainfield, N. J., assignor to Bell- .TelephoneLaboratories, Incorporated, New York, N. Y., a corporation of New YorkApplication August 17, 1949, Serial No. 110,684

This invention relates to selenium rectifiers and to methods of makingthem. a

Devices of the typeto which this invention pertains comprise, ingeneral, a metallic backin member or electrode, for example of nickel oraluminum,a layer of selenium upon one face ofithebacking member, and afront electrode, for example of a metal. such as tin, or an alloy, suchas oftin, bismuthand cadmium, upon the outer face of the selenium layer.The electrical characteristics of such devices, in the forward orconducting direction, are dependent upon three. components namely the.main rectifying barrier betweenthe selenium and the front electrode,the selenium layer and the barrier between the seleniumjand the backingmember or electroda Desirably, of' course, the resistance in the forwarddirection shouldbe low and the operating characteristics should bestable. 7 The barrier between the. selenium and the backing member,commonly refe'rred to as the secondary barrier, has a direction ofrectification opposite to that of the main barrier, that is the onebetween the selenium and the front electrode. Thus, it adds totheforward resistance. Further, it has been found that in presently knowndevices whereas the resistance of the selenium layer-remainssubstantially constant and the resistance of;the main barrier increasesbut slightly with time, the resistance of the secondary barrierincreases substantially. This results in degradation of the performanceof such devices, particularly in a substantial increase in the forwardresistance.

Prior attempts to reduce the secondary barrier resistance have involvedthe addition of a halogen, notably iodine, to the selenium or,particularly in the case of aluminum back electrodes, the provision of alayer of bismuth between the selenium and the aluminum. Although theseeffect some improvement in decreasing the secondary barrier resistance,this resistance still is subject to i'ncreasewith age. Further, otherundesirable properties maybe introduced thereby. For example, thehalogen addition increases the sensitivity of the rectifien toatmospheric moisiure and renders. the rectifier morerdifficult to orm. v.ri a

One general object of this invention is to improve the performancecharacteristics of selenium rectifiers. e i i .lVIore specifically,objects of this invention are to :reduce' the secondary barrierresistance in selenium rectifiers, to minimize variations in this.resistance with time and to realize these de- 11 Claims. (01. its-3'66)siderata concomitantly without the introduction of undesirableproperties in-the rectifiers.

Another object of this invention is to enable and f cili e. he economicmass production of selenium rectifiers having stable and low secondary;barrier resistance. i

In accordancejw" h one feature of this invention, ina selenium rectifierofthe general construction described hereinabove, a layer or film oftellurium is provided between the selenium layer and the backing member.Such aninterposed layer or film, it has been found, results not only inavery low secondary barrier 'resistance but also such a resistancewhich. is stable with time. Furthermore, rectifiers including suchlayers are readilyformed and substantially unaffected by atmosphericconditions.

A more specific feature of this invention resides in the-method ofmanufacturinga selenium rectifier which comprises roughing and cleaningan element of a material. suitable for aback electrode, vapor depositinga thin'layer of tellurium on the roughened surface and a layer ofselenium on the tellu'rium, in a vacuum, then heat treating the unit toconvert the selenium from the amorphous to the crystalline state,mounting a front contact onthe selenium surface, and electricallyforming a rectifying barrier between the front contact and the selenium.

The foregoing and other objects and features of the invention will beunderstood more=fully and clearly from the following detaileddescription of an illustrative embodiment thereof taken inconnectionfwith the appended drawings in which: M Fig. lis alongitudinal section of a vaporizing apparatus suitable, for'practicinga method in accordance with this, invention;

Fig. 2 is a cross-sectionalview of the apparatus of Fig. 1 taken alongthe line 2-1;

Fig. ,3 is a partial cross-section of t'heapparatus of Fig. 1 takenalongtheline 3-3 Fig. 4 is a plot of voltage versus current on alogarithmic scale showing a typical forward characteristic of anickel-backed selenium unit, andits development during aging, curves Aand Aa respectively, the subscript (a) being employed throughout thisspecificationflto identify the aged characteristics, including the threecomponent resistances which makeup the overall characteristic: theresistance of the ohmic body curve B, the resistanceof the main'barrier,curves C and Ca, and that of the secondary barrier, curves D and De; i h

'Fig, 5 isa logarithmic plot of voltage versus current to the same scaleas the curves of Figs. 4 and showing the forward characteristics andsecondary barrier effects of a unit of this invenmeasured characteristicis such as to reducethe forward current at one volt, (the usualoperating voltage) from 60 milliamps to milliamps for the particularunit described.

"The aluminum-selenium units also exhibit a large secondary barrier.loss, curves I and J of Fig. 5, which seriously'irnpairs' the operationof the device.

In order to more closely approach an ideal rectifier it is desirable toreduce the secondary barrier to an intimate, zero resistance, timestable contact between the selenium and the backing Y plate.

tion comprising a selenium film with atellurium interlayer on a backingof nickel before forming, curves Kr and Li respectively, the subscript fbeing employed hereafter to identify the characteristics of a unit whichhas not yet been formed, after forming, curves K and L, and after aging,curves Ka andls; I.

Fig. 7 shows a logarithmic plot of' the overall characteristics andthatof the secondary aluminum-tellurium-selenium barrier of a unitconstructed in accordance with this invention and comprising a seleniumfilm having an aluminum backing anda tellurium interlayer before formingcurves Mr and Nr, after rormingcurvesM and N, and after aging-Curves MandN and Fig; '8 isa perspective View of a selenium rectifierembo-dying'this invention withpor-tions thereof broken away. 5

-It is'not'ed that the curves whichare shown are for an average of-arn-rmber of three-quarter inc'hdiarneter washer units of the generalconstruction shownp in Fig; :8 havi-n-g a non-iodized seleniumlayer. isf Referring now'to theydrawings, -t-he device of Fig. 8"isillustrativepf those towvhichthis invention pertains; It comprises awasher ll having a roughened surface 2 0f some material such as nickelwhich provides the back electrodeand support for the remainder of theelements, a front electrode 3 andfa selenium layer 3. i It may includealso anintermediate layer 5 between the selenium and the back electrode.The front electrode 3 is ofsome material such as an alloy of tin,bismuth and cadmium applied to theunit in intimate contactwith theunderlying layer of selenium 4 to'form at the interface between the twothe main "rectifying barrier. 'The secondary rectifying barrier in priorart devices is generally formed between the selenium and the backcontact 2. Such barrier is "substantially eliminated inaccordance withthis invention by making the layer 5 of tellurium as hereinafterdescribed.

As evidenced by the-curves ofFi'g. 4, the secondary barrier contributesa considerable portion of the resistance in the characteristic. of theprior art selenium-nickel units. This secondary barrier, curves D andDa, is an undesirable part of the rectifier because it adds unwantedresistance to the unit. Furthermora'the continued development of thissecondary barrier'during its use is largely responsible for theprogressive increase in the forward resistance of the rectifier known asaging. Curves D and De. show how this secondary barrier develops duringaging and how its v pment is reflected in an increasedforward resistance"for the rectifier characteristic curve Aa. During agingthe ohmic bodyresistance remains substantially unchanged, curve B and the mainbarrier, curves C and Ca, increases only slightly in resistance. Thegreatest change is in the reverse characteristic of the secondarybarrier and the resulting increase in resistance of the Several methodshave been employed in an attempt Ito-'reducethe undesirable secondarybarrier effects; ,One' is to employ small quantities of iodine added tothe selenium in the fabrication of the nickel-backed unit, and whilethis decreases, somewhat, both the magnitude of the secondarynickel-selenium barrier and the rapidity with which it changeswith time,these units are sensitive tov atmospheridmois'ture,and are difficult toform, Where aluminum "backings are employed, the surface of'the plate iscovered with an evaporated,,coating of bisrn uth before applying theselenium. layer in" order to'reduce the secondary barrier-effects. CurveJ of Fig. :5

shows the characteristic with".the bismuth inner layer. However, thisstill results in'a,rather large 'magnitude secondary barrier W'hichhha'sa tendency to age inaboutthemannenof the nickel- .selenium barrier.

In accordance with. this.linventiomjextremely desirable characteristics;includingfa greatly reduced s'eccndary barrier resistance which .issubstantially independentiof the, support plate material, has astable'value unusually low aging, and eliminates the .needlofaddingihalogen to the selenium, thus permitting. the'un'its to form morerapi-dlyfandlh'iakir'ig the-Ihn'its more stable istics of a numbler oi-typical nickel-backed units with the -tellurium i erlayerf. Curves areshown for the unit before .forming, after forming, and after aging. Thebody resistanoe and main barrier components-of the overall resistancehave not been included in thesecurvessincethe .body resistance changesvery little-andthe-mainbarrier ages in about the same wayas does thatfor the standard unit showrr in age," Th e secondary barriercharacteristic is 'oflower-resistance than for units ho t hayingfihejtellurium layer, and [it does not changesubstantiallyjm resistance duringforming-and aging. r.Ihese; two features result in a compositecharacteristic z'ivhi'oh' is 'zoflow resistance in the one volt regionand which does not exhibit substantial increase inzresist-anceicluringaging. The advantages :realized by the use of the tellurium protectivelayer-'on-the nickel backing are evident- When:thesevnharacteristics arecompared with thosefzof Fig. '4, wherein the current at one volt afteraging is about one-fifth of that for the telluriumffilm units? 1' The.effect refusing; the tellu-rium sprotective layer technique withaluminum backinglisshoxvn in Fig. 7. The curves for the-average forwardcharacteristics. of. 1; typical 'iun'rts: before forming,

a after forming .and after :aging show again the low resistancesecondary barriercharacteristic and teristic in the oneyolt operatingregion. They show also, as was the-case for'units' made with thetellurium coated nickel backings, that the characteristics in theoperating region change onlvslightly with time.

Rectifiers may be constructed in accordance with this invention on abacking member of some suitable material such as nickel, nickel coatediron, aluminum, or magnesium which may be in any. convenient form, forexample a disc f or washer. The preliminary step in the treatment of thebacking member is to roughen the surface by sandblasting or anequivalent mechanical working so that the selenium layer is providedwitha surface to which it will adhere during the subsequent processingand use. The residue from the roughening process is then removed by apreliminary washing after which the backing member is thoroughly cleanedof all dirt and grease. This may be done by dipping the washer inhydrochloric acid for about ten seconds, then washing inwater, dippingin nitric acid for about ten seconds and again washing in water, rinsingin acetone and then rinsing in ether and drying. The backing members maythen be degassed and cleaned of residual grease and dirt, if any, byheating them in a vacuum of about millimeters of mercury at 600 or 700C. for about thirty minutes. All of the foregoing preliminary steps maybe performed on a large number of washers at the same time.

The cleaned units, washers H in Figs. 1 and 2 of the drawing, are thenmounted on a holder, for example, the rack 12 having the spring fingersl3 depending therefrom and engaging the central apertures of the washersI I. The holder l2 and the mounted washers H are then placed within avacuum chamber 14. The chamber I4 is then sealed by placing the cover I5thereon with the gasket I6 intermediate the cover and chamber wall.Atmospheric pressure on the exterior of the cover forces it against theend of the chamber wall and compresses the gasket 16 to form an airtightjoint when the chamber I4 is evacuated through the exhaust tube 18.

A pair of troughs l9 and 20 are attached to the cover l5 and extendalong the bottom of the chamber l4 below the mounted discs. Thesetroughs are supported from the cover at one end by metal straps 2| and22 which are connected to the terminal bolts 23 and 24 secured ininsulating bushings 25 and 26 on the cover. The opposite ends of thetroughs are supported by metal arms 21 and 29 depending from the off-setend of the bar 29 which is connected to the terminal 30 extendingthrough the cover [5. Additional support is provided for the troughs bythe insulating bead 31 secured to the bar 29 and functioning as a legtherefor resting on the bottom wall of the chamber.

The troughs l9 and 20 are filled with the proper amount of tellurium andselenium, respectively, prior to mounting cover I5. After the chamber [4has been evacuated to a pressure of about 10- millimeters of mercury, acoating of tellurium having a thickness of about 2 milligrams per squareinch is vapor deposited on the washers by passing a sufficient currentbetween the terminals 23 and 3|] to cause the trough I9 to heat up tothe temperature at which tellurium vaporizes. When a sufficient layer oftellurium has been deposited upon the washers the current is removedfrom the trough l9 and applied to trough 29 to vaporize the seleniumtherein. The backing washers are sufiiciently spaced from the trough 6heatersto-remaincool enough to allow condensationof the vaporized metalsthereon. The: rate of deposition of the metals should be suflicientlyrapid to avoid heating of the-washers and consequently melting of thecondensed metals. Since the electrode surface must be relatively roughto retain the selenium layer, the tellurium layer must coat the backingmember without smoothing the roughenedsurface, therefore, this layer isonly about .0005 "millimeter. in thickness while the layer of depositedselenium must be thick enough to be continuous and present a smoothsurface. A selenium layer approximately two mils in thickness has beenfound satisfactory.

'The seleniumas deposited is in the amorphous state which is of too highresistance to be practical for use in rectifiers. Therefore, it must beheat treated to convert it to the crystalline form. This treatmentcomprises a preliminary heating at about 1-10 0. for a period of aboutfour hours in air and thena high temperature treatment also in air forabout twenty minutes at a temperature in the range of about 214 C. to217 C. A front'contact is now provided for each unit by one of wellknown methods; for example, the contact may be a spray deposited layerof soft metal suchas tin or an alloysuch as one of tin, bismuth andcadmium. Forming of the body is next accomplished by passing currentthrough it in the high resistance direction, from the backing memberthrough the selenium to the soft metal front contact, underprogressively increasing voltages up to about 24 volts.

The operating characteristics, both initially and over the entire lifeof a unit constructed in accordance with this invention are considerablybetter than those of the prior art units as pointed out in the abovediscussion of the curves of Figs. 4, 5, 6, and '7. The telluriuminterlayer enables a wide choice of backing materials to be employed forselenium rectifiers while maintaining a low stable secondary barrierresistance. Further the process of manufacturing these units is amenableto mass production and requires only a minimum of handling of theindividual units.

What is claimed is:

1. An asymmetric conducting device comprising a backing electrode, athin layer of tellurium thereon, a selenium layer on said telluriumlayer, and a front electrode contacting said selenium layer.

2. An asymmetric conducting device comprising a backing electrode havinga nickel surface, a thin layer of tellurium thereon, a selenium layer onsaid tellurium layerand a front electrode contacting the surface of saidselenium.

3. An asymmetric conducting device compris ing a backing electrodehaving an aluminum surface, a thin layer of tellurium thereon, aselenium layer on said tellurium layer and a front electrode contactingthe surface of said selenium.

4. An asymmetric conducting device comprising a backing electrode havinga magnesium surface, a thin layer of tellurium thereon, a selenium layeron said tellurium layer and a front electrode contacting the surface ofsaid selenium.

5. An asymmetric conducting device comprising a backing electrode, alayer of tellurium having a thickness of the order of .005 millimeterthereon, a selenium layer on said tellurium layer, and a front electrodecontacting the surface of said selenium.

6. An asymmetric conducting device comprising a backing electrode, avapor deposited layer of tellurium on a portion of said electrode, aseleni- '7 um layer on said tellurium layer and a front electrodecontacting the surface of said selenium.

7. An asymmetric .conducting device comprising a backing electrode, alayer of tellurium thereon,-;a vapor deposited layer of selenium, and afront electrode contacting the surface of said selenium; p I

' 8. An asymmetric, conducting device-comprising a backing electrode; avapor deposited film of tellurium thereon; a vapor deposited seleniumlayer on said tellurium layer, and a front electrode contacting thesurface of said selenium.

9. The method of; making aselenium rectifier having a low, stablesecondary rectifying barrier resistance that comprises coating a surfaceof an electrode element of suitable material with a thin layer oftellurium, coating the tellurium layer with a layer of selenium, heattreating the unit to convert the selenium to the crystalline state,vapplying a front'electrode to the selenium surface and electricallyforming the'unit.

10. The method of making a selenium rectifier unit which comprisesrougheni-ng a surface of an electrode element-of suitable metal,cleaning the element, applying a thin coating of tellurium to theroughened surface of said element, coating the tellurium layer with alayer of selenium of sufiicient thickness to have .a relatively smoothexternal surface, heat treating the unit to conyert the selenium to acrystalline state, applying a front electrode to the selenium surfaceand electrically forming the unit.

11. The method of making selenium rectifier units thatcomprisesroughening a surface of each of a plurality of electrodeelements of suitable material, vapor depositing tellurium on theroughened surfaces of said cleaned elements in a vacuum, vapordepositing selenium on the tellurium coating surfaces of said elementsin a vacuum, the amount of selenium deposited being sufficient toproduce a layer that is thick enough to have a relatively smoothexternal surface, heat treating the units to convert the selenium to thecrystalline state, applying front electrodes to the selenium surface andelectrically forming the units.

JOHN N. SHIVE.

REFERENCES CITED The following references are. of record in the file ofthis patent:

, UNITED STATES PATENTS Name Date 'Brunke Feb. 6, 1940

1. AN ASYMMETRIC CONDUCTING DEVICED COMPRISING A BACKING ELECTRODE, ATHIN LAYER OF TELLURIUM THEREON, A SELENIUM LAYER ON SAID TELLURIUMLAYER, AND A FRONT ELECTRODE CONTACTING SAID SELENIUM LAYER.